In general, for avoiding and warning in advance of the accidents of running automobiles, and for further realizing automatic driving, recognition of surrounding environment where obstacles such as other vehicles are present and prediction of behaviors of the obstacles are important. For this purpose, various obstacle detecting systems have been proposed. For example, Patent Document 1 discloses a technique for detecting whether an obstacle is present around an own vehicle or not using an in-vehicle sensor such as a radar-based sensor and utilizing the detection result for prediction for collision warning, collision avoidance, adaptive cruising control, and the like.    Patent Document 1: Published Japanese Translation of PCT International Application (Kohyo) No. 2003-506785